This invention relates to an engine control apparatus and, more particularly to an apparatus for controlling the ignition timing of an internal combustion engine.
Japanese Patent Pulication No. 58-13749 discloses an engine ignition timing control apparatus including a control circuit which calculates an appropriate value for the ignition timing of the engine. The control circuit also determines one of engine operating regions where the engine is operating from a relationship programmed therein. The relationship defines engine operating region as a function of engine speed and engine load. The relationship defines a knock control unnecessary region at low engine speed or at low engine loads where almost no knock occurs, a knock control necessary region at intermediate engine speed and at high engine loads where there is a great tendency toward knock, and a knock control impossible region where knock detection is impossible due to great engine vibrations and sounds. The control circuit calculates a correction factor .beta. as a function of the knock intentity, the correction factor .beta. being used in modifying the calculated ignition timing value when the engine is operating in the knock control necessary region.
When the engine is operating with knocking in the knock control necessary region, the calculated ignition timing value is modified to retard the ignition timing so as to suppress knock by substracting the correction factor .beta. from the calculated ignition timing value. Whem the engine is operating without knocking in the control necessary region, the calculated ignition timing value is modified to advance the ignition timing so as to bring the knock intentity to an acceptable limited level by adding the correction factor .beta. to the calculated ignition timing value. When the engine is operating in the knock control impossible region, the last correction factor .beta. is used to modify the calculated ignition timing value.
One serious problem associated with such a conventional engine ignition timing control apparatus is that appropriate knock control cannot be obtained sometimes in the knock control impossible region. This particularly ture when the engine operation shifts from the knock control necessary region into the knock control impossible region immediately after the engine operation shifts from the knock control unnecessary region into the knock control necessary region. This problem is stemmed mainly from the fact the knock control necessary region expands over a relatively wide engine speed range.